The Krebs Cycle.

I’m sure that we have all heard about the Krebs Cycle before, and I’m even willing to bet that you’ve learned it before. But unfortunately, most people who learn the Krebs Cycle end up forgetting it, so that’s what I’m here for! So, without further ado, let’s dive right in and learn about the best cycle on Earth!

Before delving into the Krebs Cycle, it's worth noting that cellular respiration comprises three main stages:

1. Glycolysis

2. The Krebs Cycle

3. The Electron Transport Chain

Glycolysis breaks down glucose into two molecules of pyruvate, producing a small amount of ATP in the process. These pyruvate molecules then feed into the Krebs Cycle. The final products of the Krebs Cycle are fed into the Electron Transport Chain, which produces the majority of the cell’s ATP.

Before the Krebs Cycle officially begins, the pyruvate from glycolysis is converted into acetyl-CoA, a two-carbon molecule, a process that occurs in the mitochondrial matrix. During this conversion, one carbon from pyruvate is released as carbon dioxide, and the remaining two-carbon molecule is combined with coenzyme A, forming acetyl-CoA. This molecule then enters the Krebs Cycle.

The two-carbon acetyl-CoA combines with a four-carbon molecule, oxaloacetate, to produce citrate, a six-carbon molecule. This is the first step in the cycle, and citrate is subsequently processed in several steps to regenerate oxaloacetate.

As citrate progresses through the cycle, two carbon dioxide molecules are released in separate reactions. These carbon atoms originally came from the glucose molecule that entered glycolysis. The cycle's primary function is not to produce ATP directly but rather to generate high-energy carriers: three NADH molecules, one FADH2 molecule, and one ATP (or GTP) per cycle turn.

The high-energy carriers NADH and FADH2 play pivotal roles in cellular respiration. They are produced as citrate and its derivatives are oxidized in the Krebs Cycle. These carriers then proceed to the Electron Transport Chain, where they are further oxidized, releasing their stored energy to power the production of ATP.

The final steps of the Krebs Cycle ensure that oxaloacetate is regenerated, allowing the cycle to perpetuate. This regenerated oxaloacetate can then combine with another Acetyl-CoA molecule, continuing the cycle.

The Krebs Cycle is a marvel of metabolic efficiency. For each molecule of Acetyl-CoA entering the cycle, two molecules of carbon dioxide are released, one molecule of ATP is produced directly, and multiple high-energy carriers are formed. These carriers can create even more ATP through the Electron Transport Chain.